Nonreactive refractory separating coatings for electrical steels

ABSTRACT

AN AQUEOUS SLURRY COMPRISING ABOUT 60% BY WEIGHT OF AL2O3, BASED ON THE WEIGHT OF THE WATER, AND SMALL AMOUNTS OF POLYVINYL ALCOHOL AND AN ORGANIC SUSPENDING AGENT (MORE PARTICULARLY, A CELLULOSIC SUSPENDING AGENT), IS APPLIED TO ELECTRICAL SHEET STEEL AND DRIED THEREON TO PROVIDE A RELATIVELY THIN AND UNIFORM SEPARATING COATING. A PLURALITY OF SUCH COATED SHEETS MAY BE STACKED OR A SINGLE SUCH SHEET MAY BE COILED FOR FINAL ANNEALING AT ELEVATED TEMPERATURE.

United States Patent C) 3,794,520 NONREACTIVE REFRACTORY SEPARATING COATINGS FOR ELECTRICAL STEELS Norman M. Pavlik, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa. No Drawing. Continuation-impart of abandoned application Ser. No. 22,683, Mar. 25, 1970, which is a division of application Ser. No. 681,026, Nov. 6, 1967, now Patent No. 3,523,837. This application Mar. 30, 1972, Ser. No. 239,755

Int. Cl. H01f 1/04 US. Cl. 117-230 3 Claims ABSTRACT OF THE DISCLOSURE An aqueous slurry comprising about 60% by weight of A1 based on the weight of the water, and small amounts of polyvinyl alcohol and an organic suspending agent (more particularly, a cellulosicsuspending agent), is applied to electrical sheet steel and dried thereon to provide a relatively thin and uniform separating coating. A plurality of such coated sheets may be stacked or a single such sheet may be coiled for final annealing at elevated temperature.

CROSS REFERENCE TO RELATED APPLICATION The present application is filed as a continuation-inpart of application Ser. No. 22,683 filed Mar. 25, 1970 nowabandoned which was a division of application Ser. No. 681,026, filed Nov. 6, 1967, now Pat. No. 3,523,837. See also application Ser. No. 22,682, filed Mar. 25, 1970 now US. Pat. No. 3,671,335.

BACKGROUND OF THE INVENTION It is common practice in annealing electrical sheet steel to stack a plurality of sheets or to coil a single sheet so that lots Weighing several hundreds of pounds may be annealed at a time in a furnace of suitable size. In order to prevent adjacent sheets of the stack or the adjacent convolutions of a coil from sticking together, a refractory material is introduced between the adjacent sheets or convolutions to separate them from each other. The problem of obtaining and maintaining this desired separation, and thus avoiding sticking, increases in difficulty with increasing annealing temperatures. The refractory separating medium, in some instances, is required to be essentially inert and nonreactive with respect to the alloy sheet undergoing the annealing operation. Certain textures or orientations are obtained in alloys by surface energy mechanism. For this mechanism to operate successfully, the alloy surfaces must be mirror-bright and a reaction between the separating medium and the alloy is not permissible.

This problem of reactivity is particularly important in the annealing of iron-silicon electrical sheet steels having the (100) [001] grain orientation. A process for making iron-silicon alloy sheets having this orientation is set forth in US. Pat. No. 3,240,638, issued Mar. 15, 1966 to G. W. Wiener et al. In this so-called cube textured sheet steel, the desired orientation is obtained by means of surface energy phenomena and it is imperative that the surface of the iron-silicon alloy sheet steel be extremely clean with a mirror-bright surface. A coating which reacts with the surface of the alloy sheet steel will defeat the necessary surface energy phenomena and the desired grain orientation will not be obtained.

Another group of materials in which surface energy phenomena is important in obtaining a desired orientation, and therefore, in which a nonreactive separating medium is necessary, are the alloys disclosed in application Ser. No. 372,693, filed June 4, 1964 and entitled Process for Producing Magnetic Sheets With Cube-on- Face Grain Texture by Robert G. Aspden now US. Pat. No. 3,351,501.

One material which has been successfully employed as a separating medium is a dry alumina (A1 0 powder which is sprinkled on the surface of the sheets prior to annealing.

The problem of an even distribution of powder materials of this type, and the further problem of maintaining an even distribution once it is achieved, are readily apparent. Further, it has been found that individual steel sheets in stacks employing dry alumina powder tend to shift and slide, due to the ball-bearing action of the alumina powder, and it is difficult to maintain the stack alignment. The problem of maintaining a uniform distribution of dry alumina powder in a coil of electrical steel has not been solved and further, a phenomenon known as telescoping is frequently observed in such coil as the inner turns of the coil telescope outwardly on the low friction surface provided by the alumina powder.

Ideally then, the alumina powder should be applied in a form which would adhere to the steel sheet before and during annealing, but which would be easily removable after the annealing treatment. A slurry containing alumina powder would appear to offer a solution, however, difiiculties involving the reactivity of the coating with the steel sheet during annealing have thus far prevented the successsful application of the alumina in slurry form. In particular, efforts to provide an inert slurry containing alpha phase alumina made by the Bayer process have been unsuccessful due to reactivity of the alumina with the steel sheet.

SUMMARY OF THE INVENTION This invention is an improvement in the process of heat treating coiled or stacked electrical steel sheet at elevated temperatures which involves providing the steel sheet with an adherent nonreactive refractory separating coating which is readily removable after the heat treatment.

More specifically, the coating of refractory material is applied in the form of a slurry containing fine A1 0 powder as a solid constituent suspended in an aqueous solution of polyvinyl alcohol and a cellulosic suspending agent such as hydroxyethyl cellulose. When the slurry is dried it provides a thin coating on the electrical steel sheet of adherent A1 0 powder associated with certain organic residues.

The refractory slurry of this invention contains about 30 to and preferably about 60% by weight of fine A1 0 powder suspended in an aqueous solution comprising, by weight, from about 0.5% to 4% polyvinyl alcohol and from about 0.3% to 1% hydroxyethyl cellulose. All weights stated are based on the weight of water present. When the slurry is dried on the steel sheet the weight of the coating per unit area will be from about 0.3 gm./ft. to 3 gm./ft.

It is an object of this invention to provide an adherent nonreactive refractory coating for use in the annealing of silicon-iron and other alloys annealed at high temperatures, and which coating is readily removable after annealing.

It is another object of this invention to provide a nonreactive refractory and adherent coating as a separating medium during the high temperature annealing of siliconiron alloy sheet which promotes the development of a high degree of cube texture and which is readily removable after annealing.

Other objects and advantages of the invention will, in part, be obvious and will, in part, appear hereinafter.

3 DESCRIPTION OF THE PREFERRED EMBODIMENTS The slurry of this invention is composed of alumina powder, hydroxyethyl cellulose, polyvinyl alcohol and water.

Suitable alumina powder may be produced using hydrated alumina (Al(OH) by the following process: (1) Hydrated alumina powder is placed in ceramic trays or crucibles in a layer not over 4 inches deep. The hydrated alumina powder is alpha phase alumina. (2) The trays or crucibles of hydrated alumina are placed in an oven at a temperature of about 100 C. and held approximately 2 hours. Thereafter the temperature is raised about 100 C. at a time with holding periods of at least 3 hours at about 200 C. and about 300 C. The temperature of the oven is then increased to about 500 C. and a holding period at that temperature of 12 hours is employed. The temperature of the oven is finally raised to about 1000 C. and a holding period of 12 hours is employed. This curing schedule has proven satisfactory, but other schedules may clearly be used, the temperature rise at each step is limited only to prevent excessively violent boiling off of the water in the alumina, with consequent loss of powder. Approximately 35% by weight of the hydrated alumina, namely the water of hydration must be removed.

Using nominal 325 mesh alumina trihydrate, the particle size distribution of alumina produced by this method was analyzed as follows:

The alumina processed as described was identified by X- ray diffraction means as being kappa (K) phase aluminum oxide (ASTM 4-878).

Hydroxyethyl cellulose is a non-ionic water-soluble polymer derived from cellulose, one type of which is sold under the trademark, Natrosol. In the slurry of this invention the hydroxyethyl cellulose performs the function of a suspending agent. Hydroxyethyl cellulose is a white free-flowing powder and one preferred aqueous solution of this material is prepared initially in the following manner: (1) A predetermined amount of water is heated to a temperature of 60 C.; (2) exactly 0.6% hydroxyethyl cellulose based on the amount of water is weighed out; (3) the heated water is stirred to form a vortex and the hydroxyethyl cellulose is slowly sifted into the vortex of vigorously agitated water. The rate of addition of the hydroxyethyl cellulose should be slow enough for the particles to separate in water without lump formation, but not so slow that the solution thickens appreciably before all of the solid is added. Agitation should be continued until all of the swollen or gelatinized particles are dissolved to yield a smooth solution; (4) the solution of hydroxyethyl cellulose is cooled to room temperature.

The hydroxyethyl cellulose is available in grades of high, medium and low molecular weights. In general, it is desired in the present invention to provide the required viscosity with as small an addition as possible, and for this purpose, the high molecular weight compounds are best. Those compounds which produce a solution viscosity of from 300 to 500 centipoises at 25 C., and preferably from 350 to 400 centipoises, with an addition of up to 1%, by weight, or less of the compound, are satisfactory.

Somewhat less satisfactory as suspending agents, although still useful, are the high molecular Weight compounds of carboxymethyl cellulose.

The polyvinyl alcohol (PVA) is added to the slurry primarily to improve the adhesion of the coating to the sheet steel, but it also contributes secondarily as a suspending agent. The preferred polyvinyl alcohol employed is a high molecular weight type having a molecular weight in the range of 73 to 125. Such a product is sold under the trademark Elvanol. A solution of polyvinyl alcohol is prepared in the following manner: (1) Water at a temperature of 75 F. or lower is stirred with a high speed agitator to produce a vortex. (2) The polyvinyl alcohol powder is sifted directly into the vortex so that it is rapidly wet and dispersed and the slurry thus formed is stirred for 10 minutes. (3) The polyvinyl alcohol slurry is heated to 195 F. or higher and agitated at this temperature until solution is complete, which usually takes from 30 to 60 minutes. The suggested maximum concentrations for solutions prepared with ordinary high speed mixers is from 10 to 15% by weight of polyvinyl alcohol having a molecular weight of 73 to 125. (4) The polyvinyl alcohol solution is reduced to about 1% by weight of PVA by the addition of water.

While polyvinyl alcohols (PVA) of high molecular weight are preferred, polyvinyl alcohols of intermediate and low molecular weight may be used, but larger additions of up to 4% must be employed to obtain the same result accomplished by smaller additions of high molecular weight PVA.

In the preparation of the coating slurry, the separately prepared hydroxyethyl cellulose solution and the polyvinyl alcohol solution are mixed together. Then, for example, about 60% by weight of finely divided A1 0 based on the weight of water to be used, which has had the water of hydration removed as described above, is added to the hydroxyethyl cellulose-polyvinyl alcohol solution while it is being vigorously stirred. The A1 0 is added slowly to the vortex to assure that all particles are suspended. The typical slurry mix has the following constituents:

1000 parts by weight H 0 6 parts by weight hydroxyethyl cellulose 10 parts by weight polyvinyl alcohol 600 parts by weight A1 0 (average particle size 200 mesh and finer) The alumina slurry is applied to the electrical steel sheet by coating rolls, but in order to obtain a uniform coating with the alumina slurry, it is essential that the alumina remain in suspension. A solution viscosity of at least 300 to 500 centipoises, and preferably 350 to 400 centipoises (at 25 C.), tends to keep the solids in suspension. The organic additives present in the slurry, as described above, aid in achieving a viscosity in this range and so help maintain the solids in suspension and further, these organic additives promote adhesion of the alumina particles to the alloy sheets. However, these additives can be used only in relatively small quantities to avoid introducing excessive amounts of carbonaceous contaminants into the annealing process. Accordingly, continuous agitation of the slurry is desirable, and a mixing tank with suitable slurry pumps may be used to aid in holding the slurry in suspension and to deliver it to the coating rolls. The electrical sheet, of course, passes between the coating rolls and a slurry reservoir is provided to wet the bottom roll and a transfer pump from the mixing tank may be used to wet the top roll with the required amount of slurry.

The roll apparatus consists of two mated rubber or plastic rolls with a screwdown device to regulate the roll pressure. The rolls are grooved .008 to .012 inch deep with about 36 threads per inch and good results are obtained when the durometer hardness of the rolls is about 50. The length and diameter of the rolls is determined by the width and length of the electrical steel sheets, and the velocity of the sheet.

Following the coating step, the sheet is passed through a drying oven to obtain a temperature in the electrical steel sheet of between and C. The temperature is maintained in this range since higher temperatures may char the additives.

It is essential that an extremely uniform coating be obtained on the electrical steel sheet with no trailing Gm./ft. Minimum 0.3 Maximum 3.0

Excellent results have been obtained when the amount of dried slurry present on the electrical steel sheet was approximately 0.4 gm./ft. of the total surface area. While the dried alumina coating is reasonably adherent to the electrical sheet steel, care must be exercised in handling and in stacking the dried sheets. The sheets or-laminations should not be rubbed or scraped in stacking or coiling because any bare sheet surfaces exposed by such scraping may result in welding together of the sheets or coil convolutions.

One satisfactory procedure to avoid scraping or rubbing the laminations prior to the transformation anneal is to stack a plurality of laminations between V: inch thick plates of low carbon steel to eliminate individual handling of the laminations or sheets. The stack height of the sheets maybe 6 inches, for example; the sheets having a thickness of from 3 to 15 mils. The stack of plates and electrical sheet laminations can be banded tightly and stored in a dry condition prior to the transformation anneal.

Coils of electrical sheet steel can also be banded and stored under dry conditions prior to the transformation anneal.

The transformation anneal of the coated steel is carried out at about 1200 C. in a dry hydrogen atmosphere. Under these conditions the organic binding and suspension agents present in the alumina coating are completely removed. The alumina remains on the surface of the electrical steel sheet. It is inert and can be removed by dry brushing or wiping the surface with a cloth after the transformation anneal. A very clean surface on the electrical steel sheet is then exposed.

' Example Percent Alumina 60 Hydroxyethyl cellulose 0.6 Polyvinyl alcohol 1 The sheets coated with this slurry are placed on a conveyor belt which carries them through a gas fired oven. In

the oven, the sheets attain a temperature of about 150 C. which thoroughly dries the coating. The dried coating adheres well to the alloy sheets and is present on the sheets in the amount of 0.35 gm./ft. on each side of a sheet.

The coated sheets are stacked and annealed in a furnace at a temperature of 1200 C. for a period of 24 hours in an atmosphere of dry hydrogen (dew point of 60 C.) into which is introduced 35 ppm. of hydrogen sulfide. The alloy sheets are thereafter removed from the furnace and cooled to room temperature. No sticking is observed between sheets and the alumina is readily removed by light brushing. Transformation of the alloy sheets to cube texture by the described annealing treatment is accomplished to the extent of 98% (100) [001] texture by volume.

The success achieved with the process described above is in striking contrast to the man elforts previously made to anneal this electrical sheet steel with Bayer process alpha phase alumina applied as a slurry. In these previous efforts, reaction of the alumina with the sheet was sufiicie'nt to inhibit transformation to cube texture.

The teachings of the present invention have been employed in producing cube-on-face texture material and for comparison purposes, substantially identical samples were prepared employing a process of the prior art. Samples of a nominal 3% silicon steel were employed as the underlying material and a slurry was applied to the surface thereof employing kappa phase alumina, polyvinyl alcohol and hydroxyethyl cellulose in accordance with the teachings set forth hereinbefore. For comparison purposes, high grade bauxite mineral is employed as the source of A1 0 The composition of bauxite varies from deposit to deposit in the ranges of 30% to 70% A1 0 9% to 31% H 0, 3% to 25% Fe O 2% to 9% SiO and 1% to 3% TiO along with small amounts of various impurities. The water that is associated with these ores is normally removed by calcining. a high grade bauxite thus will contain after calcining at least 86% A1 0 The samples of bauxite employed herein were received in the form of course particles partially calcined. The materials were ball-milled to form a fine powder which had a reddish-brown color. The bauxite samples were then calcined at about 1200 C. in air to form alpha-phase alumina. After this treatment, the powders had a very light gray appearance. Thereafter the powders were heated for 16 hours at 1200 C. in dry hydrogen to reduce any oxides that might otherwise be reduced during the cube texture anneal. The samples as thus annealed turned a darker gray indicating the reduction of some oxides.

The calcined bauxite material which contained a minimum of 86% alpha phase A1 0 was suspended in water together with the amount of polyvinyl alcohol and hydroxyethyl cellulose as utilized in the method of the present invention. The slurry was applied to duplicate samples of silicon steel which were thereafter annealed at a temperature of 1200 C. for a period of 24 hours in an atmosphere of dry hydrogen. Thereafter, the materials, after cooling to room temperature, were tested for both their DC and their AC magnetic characteristics. The results are set forth hereinafter in Table I.

TABLE I DO magnetic data AC magnetic data Magne- Flux Flux tizing density density Core force K 11 loss, oersted gausses gausses Watts/ft Invention 1. 0 15. 34 15 10. 0 17. 25 17 1. 17

of 14.35 and 16.25 kilogausses respectively. Thus it is seen that the teachings of the present invention produce far superior magnetic characteristics. F-S in Table I refers to Foster et al. US. Pat. No. 3,282,747, Example V.

Substantially similar results were obtained when a comparison of the AC magnetic test data is compared for the core loss values both at a flux density 15 and 17 kilogausses. A substantial improvement is noted in the core loss when processed in accordance with the teachings of the present invention in comparison with prior art teachmgs.

The slurry of the invention has also been employed in the annealing of 50% nickel-50% iron alloy. The alloy strip is coated on one side with the slurry and stainless steel wiper bars are used to obtain a uniform coating. The

7 coating is dried 'by passing the strip througha .bankof heat lamps before the strip is wound into cores. The wound cores are than annealed and it is found that the coating is nonreactive at the annealing temperature, typically 1066 There has thus been presented means for obtaining, a substantially completely inert slurry coating on siliconiron electrical sheet steel as a separating medium during the transformation anneal of such electrical steel sheets to obtain a cube texture. The slurry coating described herein provides substantial processing advantages over dry dusting or dusting over a wet surface.

It will be understood by those skilled in the art that although the invention has been described in connection with preferred embodiments, modification and variations in composition and in processing schedule and in other aspects, the invention may be employed without departing from the underlying spirit and scope of the invention.

1. An electrical steel sheet having an adherent coating thereon which coating is non-reactive with the steel sheet during transformation annealing at a temperature in excess of 1000" C., said coating consisting essentially of the product of evaporation of a slurry containing from 30% to 70% by weight of kappa phase alumina particles suspended in an aqueous solution of from 0.5% to 4% by weight of polyvinyl alcohol and from 0.3% to 1% by weight of at least one of the group consisting of hydroxyethyl cellulose and carboxymethyl cellulose and mixtures thereof, all weights being based upon the amount of water present, said kappa aluminahaving been derived b, the systematic removal of the water ofhydration fro m Anon 1 I References Cited: UNITED STATES PATENTS Robinson 117-1'27- X 2,641,556." 6/1953 2,871,143 1/1959 Getting, Jr. l'17-I27 2,985,855 -5/l961 Stone 148- 113 X 3,282,747 11/1966 Foster et'al. 117"127 X 3,389,006 6/1968 Kohler '117127 X 3,459,602 8/1969 Mueller 14828'X RALPH s. KENDALL, Primary Examiner 3 Us. c1. X.R. 

